Informational Message: The current Metropolis proposal is about to be rejected because of the following issue:
Exception: Exception: multiply: A[7] is nan, but must not be nan! (in ‘/Users/me/workspace/myproject/mymodel.stan’ at line 174)

The 7th element of the matrix on the left being multiplied and you have to test for that using is_nan with a for loop over the elements of the matrix. Also, there should be a more specific line number pointing to multiplication inside your iQt function.